Bourton fresh water tank

When looking at a 2015 Bourton last year I was surprised to see a water tank under the offside settee. The salesman called this a header tank but thought that it would no longer be present in the 2016 model. I think this inside tank holds 10 litres. I thought the salesman was probably correct about the 2016 model not having this internal tank because the earlier model had a fresh water capacity of 72 litres and the 2016 model's capacity is 62 litres but when I had an actual little nosey around the new model, the internal tank was still there, taking up space!
Does anyone know the purpose of this tank and why there is the difference in fresh water capacities between the 2015 and 2016 models since the internal tank is still present?